The invention relates to a condensation dryer having a heat pump and with detection of an impermissible operating state, and to a preferred method for operating the dryer.
In a condensation dryer, air (what is termed process air) is ducted by a fan across a heater into a drum as a drying chamber containing damp laundry items. The hot air absorbs moisture from the laundry items requiring to be dried. Having passed through the drum, the then moist process air is ducted into a heat exchanger upstream of which as a rule a lint filter is connected. The moist process air is cooled in said heat exchanger (for example an air-air heat exchanger or a heat pump's heat sink) so that the water contained in the moist process air condenses. The condensed water is then generally collected in a suitable container and the cooled and dried air ducted back to the heater (which may be a heat pump's heat source) and then to the drum.
That drying process is in certain circumstances very energy-intensive because the cooling-air current heated in the heat exchanger as the process air is cooled can in energy terms be lost to the process. That energy loss can be significantly reduced by employing a heat pump. In the case of a condensation dryer fitted with a heat pump the warm, moisture-laden process air is cooled substantially in a heat sink of the heat pump, where the heat extracted from the process air is used for, for example, evaporating a coolant employed in the heat-pump circuit. The heat absorbed in the heat sink is transported inside the heat pump to the heat source and there given off again—possibly at a temperature raised above that at the heat sink. In a heat pump, which operates with a coolant as the heat-transporting means, with the coolant being evaporated in the heat sink and condensed in the heat source, via a compressor the evaporated, gaseous coolant reaches the heat source, which can here be designated a condenser, where, owing to the gaseous coolant's being condensed, heat is released that is used for heating the process air before it enters the drum. The condensed coolant finally flows back to the evaporator through a throttle; the throttle serves to reduce the internal pressure in the coolant so it can evaporate in the evaporator with heat again being absorbed. The heat pump that is operated in such a way with a circulating coolant is known also as a “compressor heat pump”. Other heat-pump designs are also known.
DE 40 23 000 C2 discloses a laundry dryer that has a heat pump and arranged in which in the process-air duct between the condenser and evaporator is an incoming-air orifice that can be sealed with a controllable sealing device.
DE 197 28 197 A1 discloses a method for detecting unacceptable operating conditions in a laundry dryer as well as a corresponding laundry dryer. The aim of the method is to enable separate or joint recording of different operating conditions of too high temperature that originate in different regions. The temperature is recorded periodically in the supply-air current above a supply-air heater and before the laundry drum, a difference value or gradient is created from two successively recorded values, said difference value (gradient) is compared with a preset difference value (gradient), with—if the newly created difference value is greater in absolute terms than the preset difference value—a counting value being raised by a step, said value being compared with a preset value, and—if the current value is greater than the preset value—the laundry-dryer heater being switched off and/or an operating-condition display activated.
WO 2008/086933 A1 discloses a condensation dryer having a drying chamber, a process-air circuit in which a heater for heating the process air is located and the heated process air can be ducted across the items requiring to be dried by means of a fan, an air-air heat exchanger, and a heat-pump circuit having an evaporator, a compressor, and a condenser. Located in the heat-pump circuit between the condenser and evaporator is an additional heat exchanger that is functionally coupled to the air-air heat exchanger. The temperature of the heat pump's coolant, particularly in the condenser, is kept within the permissible range via the heat pump's controller and the additional heat exchanger. Temperature sensors are furthermore employed for regulating the temperature of the coolant or heat pump and the temperature of the process air in the heat-pump circuit and/or process-air circuit.
DE 29 17 230 A1 describes a method for controlling the operation of a drying apparatus having a drum for drying an article, with which method a desired degree of dryness to be attained by means of the apparatus for the article requiring to be dried is programmed, a multiplicity of temperatures of the heated air entering the drum and of the moisture-laden air leaving the drum is registered, a maximum temperature difference between the temperatures of the heated air and the temperatures of the moisture-laden air is determined during each drying cycle of the apparatus, and as a function of the maximum temperature difference and the programmed degree of dryness for the article a final temperature difference between the temperatures is derived that is a measure of the programmed degree of dryness, and the machine function is switched off when the final temperature difference is present.
EP 1 593 770 A2 describes a clothes dryer having a drying chamber, a heat-pump mechanism in which a coolant can circulate between a heat absorber, a compressor, a throttle unit, and a heat radiator, and an air circulation path for circulating drying air from the drying chamber through the heat absorber and heat radiator back to the drying chamber. An air discharge part is located in the air circulation path between the drying chamber and heat absorber so that a part of the drying air flowing along the air circulation path from the drying chamber to the heat absorber will be conveyed to the outside through the air emptying part. In the embodiment variant of the laundry dryer shown in FIG. 10 the temperature of the coolant is measured and regulated in such a way as to keep within a prespecified range.
The traditionally employed air-air heat exchanger—operated in crossover or counterflow mode—and the electric heater are generally completely replaced with a heat pump. Compared with a dryer having an air-air heat exchanger and a resistance heater it is possible thereby to achieve a 20-50% reduction in the energy required for a drying process.
A compressor-heat pump as a rule operates optimally within specific temperature ranges in the evaporator and the condenser. What is problematic about using a compressor-heat pump in the condensation dryer is the usually high temperature in the condenser, which for process reasons can result in its no longer being possible to condense or fully condense the coolant; the compressor will then have to be switched off and/or a substantial impairment in the heat pump's effectiveness will have to be accepted. That problem is even worse when the compressor is supported by an additional heater in the process-air circuit to achieve faster heating of the process air and hence shorter drying times. Moreover, the circulating process air can be impeded by soiled air paths. That can likewise cause the temperature of the coolant to rise. Operating states of such kind can result in damage to the heat pump or other parts of the dryer and so are impermissible.
In a conventional dryer an impermissible operating state, for example a reduced circulation of the process air (reduction in air performance) is ascertained by registering a temperature in the process-air current above a heater for the process air and in front of the drying chamber at regular intervals and forming from in each two successively registered values a difference value corresponding to a time gradient. That information generally does not have to be available in that form in the case of a dryer fitted with a heat pump (a heat-pump dryer). For example in a heat-pump dryer the heat pump is frequently sited further from the drying chamber than is the heater in a conventional condensation dryer. In any event, an impermissible operating state in a condensation dryer fitted with a heat pump can only be detected imprecisely in that way.